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-<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-module-abstractions">
-  <title>Abstractions</title>
-  <para>
-    If you find yourself repeating yourself over and over, it’s time to
-    abstract. Take, for instance, this Apache HTTP Server configuration:
-  </para>
-  <programlisting language="bash">
-{
-  services.httpd.virtualHosts =
-    { &quot;blog.example.org&quot; = {
-        documentRoot = &quot;/webroot/blog.example.org&quot;;
-        adminAddr = &quot;alice@example.org&quot;;
-        forceSSL = true;
-        enableACME = true;
-        enablePHP = true;
-      };
-      &quot;wiki.example.org&quot; = {
-        documentRoot = &quot;/webroot/wiki.example.org&quot;;
-        adminAddr = &quot;alice@example.org&quot;;
-        forceSSL = true;
-        enableACME = true;
-        enablePHP = true;
-      };
-    };
-}
-</programlisting>
-  <para>
-    It defines two virtual hosts with nearly identical configuration;
-    the only difference is the document root directories. To prevent
-    this duplication, we can use a <literal>let</literal>:
-  </para>
-  <programlisting language="bash">
-let
-  commonConfig =
-    { adminAddr = &quot;alice@example.org&quot;;
-      forceSSL = true;
-      enableACME = true;
-    };
-in
-{
-  services.httpd.virtualHosts =
-    { &quot;blog.example.org&quot; = (commonConfig // { documentRoot = &quot;/webroot/blog.example.org&quot;; });
-      &quot;wiki.example.org&quot; = (commonConfig // { documentRoot = &quot;/webroot/wiki.example.com&quot;; });
-    };
-}
-</programlisting>
-  <para>
-    The <literal>let commonConfig = ...</literal> defines a variable
-    named <literal>commonConfig</literal>. The <literal>//</literal>
-    operator merges two attribute sets, so the configuration of the
-    second virtual host is the set <literal>commonConfig</literal>
-    extended with the document root option.
-  </para>
-  <para>
-    You can write a <literal>let</literal> wherever an expression is
-    allowed. Thus, you also could have written:
-  </para>
-  <programlisting language="bash">
-{
-  services.httpd.virtualHosts =
-    let commonConfig = ...; in
-    { &quot;blog.example.org&quot; = (commonConfig // { ... })
-      &quot;wiki.example.org&quot; = (commonConfig // { ... })
-    };
-}
-</programlisting>
-  <para>
-    but not <literal>{ let commonConfig = ...; in ...; }</literal> since
-    attributes (as opposed to attribute values) are not expressions.
-  </para>
-  <para>
-    <emphasis role="strong">Functions</emphasis> provide another method
-    of abstraction. For instance, suppose that we want to generate lots
-    of different virtual hosts, all with identical configuration except
-    for the document root. This can be done as follows:
-  </para>
-  <programlisting language="bash">
-{
-  services.httpd.virtualHosts =
-    let
-      makeVirtualHost = webroot:
-        { documentRoot = webroot;
-          adminAddr = &quot;alice@example.org&quot;;
-          forceSSL = true;
-          enableACME = true;
-        };
-    in
-      { &quot;example.org&quot; = (makeVirtualHost &quot;/webroot/example.org&quot;);
-        &quot;example.com&quot; = (makeVirtualHost &quot;/webroot/example.com&quot;);
-        &quot;example.gov&quot; = (makeVirtualHost &quot;/webroot/example.gov&quot;);
-        &quot;example.nl&quot; = (makeVirtualHost &quot;/webroot/example.nl&quot;);
-      };
-}
-</programlisting>
-  <para>
-    Here, <literal>makeVirtualHost</literal> is a function that takes a
-    single argument <literal>webroot</literal> and returns the
-    configuration for a virtual host. That function is then called for
-    several names to produce the list of virtual host configurations.
-  </para>
-</section>